Generally, in a rolling device such as a rolling bearing, a rolling movement is performed between a rolling element, an outer member and an inner member that constitute a rolling device, and the raceway surfaces of the inner member and the outer member as well as the rolling contact surface of the rolling element receive repeated contact stress. Therefore, the properties required of a material constituting the inner member, outer member and rolling element include hardness, load endurance, a long rolling fatigue life, and favorable wear resistance with respective to sliding.
Thus, as the material constituting these members, SUJ2 according to the Japanese Industrial Standards is generally used as bearing steel, SUS440C according to the Japanese Industrial Standards or 13Cr martensitic stainless steel are generally used as stainless steel, and steels equivalent to SCR420 of the Japanese Industrial Standards are generally used as case hardening steels. To obtain the required properties such as rolling fatigue life, the material is subjected to quenching and tempering in the case of bearing steel or stainless steel, and in the case of case hardening steel the material is subjected to quenching and tempering after carburizing treatment or carbonitriding treatment to acquire a hardness from HRC 58 to HRC 64.
It is known that the rolling fatigue life of a rolling device such as a rolling bearing, a ball screw, a linear guide or a linear motion bearing is closely related to the lubrication state in addition to the aforementioned hardness. As used herein, the term “rolling fatigue life” refers to the total number of rotations until the material wears down and flaking appears on a part of the surface thereof due to the rolling contact surface or raceway surface of the rolling device receiving repeated stress accompanying rotation.
The quality of the lubrication state of the rolling contact surface is represented by an oil film parameter Λ (refer to the formula below) as the ratio between the oil film thickness to be formed and surface roughness, where a larger value for Λ indicates a better lubrication state. More specifically, when Λ is large, surface origin flaking caused by contact between microspikes on the surfaces is less liable to occur and the life is mainly determined by the cleanliness, hardness, material properties, heat treatment and the like of the material. Conversely, it is known that the smaller the value of Λ, the more likely the occurrence of seizure, peeling damage and surface origin flaking caused by contact between microspikes on the surfaces, and greater the reduction in the life.
Λ=h/σ
h: EHL oil film thickness
σ: composite surface roughness (σ12+σ22)1/2 
σ1, σ2 represent the roughness of two contacting surfaces (root-mean-square roughness)
This will now be described in further detail taking the example of a radial needle bearing. In a planetary gear bearing that bears a planetary gear that is widely used in transmissions and the like, since a helical gear is generally used so that transmission of force from a planetary gear as an outer member is performed smoothly, as a result of the power relationship the running track of a planetary shaft as an inner member becomes distorted. Consequently, an uneven force acts on a needle roller that is disposed between the planetary gear and the planetary shaft resulting in the occurrence of an edge load or skewing or the like, and thus the life of the bearing decreases and smearing or seizure is liable to occur.
To counteract this problem, conventionally, crowning has been performed for the needle roller to alleviate the edge load, or to prevent skewing the circumferential clearance and radial internal clearance are precisely controlled as a method to suppress the occurrence of skewing before it occurs.
Meanwhile, from the viewpoint of enhancing fuel consumption of engines accompanying CO2 emissions controls, in combination with lowering the viscosity of lubricating oils in order to increase torque at a time of high speed rotation, improved seizure resistance at a time of high speed rotation or improved durability under conditions of lean lubrication are being increasingly demanded for rolling devices. To ensure the lubricity of a needle roller bearing in this kind of usage, for example, the needle roller is provided in multiple rows and a lubrication hole is provided from the shaft end to a position between the needles of the planetary shaft as an inner member, and a lubrication-hole lubricating system is employed that performs lubrication through the lubrication hole. However, when the oil amount is insufficient, there is a risk of a seizure or the occurrence of reeling damage on the rolling contact surface.
As technology to prevent this kind of peeling damage, an automatic transmission bearing has been disclosed (see Patent Document 1) in which a ratio RMS(L)/RMS(C) between axial direction surface roughness RMS(L) and circumferential direction surface roughness RMS(C) is 1.0 or less, a parameter SK value that indicates the degree of distortion in the distribution curve of surface roughness is made a minus value, and the proportion of the surface area occupied by dents is 10 to 40%.
Further, as technology for preventing seizure, a mechanical component provided with a sliding surface that makes a sliding contact action while receiving a thrust load is disclosed in which innumerable independent micro-dents are randomly provided on the sliding surface, the surface roughness of the surface on which the micro-dents are provided is Rmax 0.6 to 2.5 μm, the parameter SK value for surface roughness is −1.6 or less, the mean surface area of micro-dents is 35 to 150 μm2, and the proportion of the surface area occupied by micro-dents on the surface is 10 to 40% (see Patent Document 2).
Next, a description is given regarding a ball screw. A ball screw used in a motor-driven injection molding machine or an electrical pressing machine or the like is used at a short stroke at which a heavy load is instantaneously applied, and is used under conditions of reciprocating motion in which the ball screw rotates in reverse after temporarily stopping in a state in which the maximum load acted. For this reason, there is a tendency for the oil film on the ball rolling contact surface to be scraped off, and for it to be difficult for a lubricant to enter the contact surface between the screw groove and the ball, resulting in insufficient oil film formation. There is thus a problem that flaking and abrasion due to surface damage are liable to occur on the rolling contact surface of the ball, the nut and the screw shaft.
In particular, damage is noticeable on contact surfaces of balls that contact with each other when relative slipping occurs at a speed that is double the rolling speed. Further, deformation of the machine stand due to the action of a heavy load or misalignment at the time of fitting make the aforementioned clashing between the balls even more noticeable and reduce the life further.
As a ball screw that can be used for this kind of usage, a ball screw has been disclosed in which fine particles of molybdenum disulfide are injected onto at least one sliding-contact portion among a screw groove, a nut and a rolling element and adhered thereto by collision so as to form a lubricant film with a film thickness of 0.5 μm or less (see Patent Document 3) thereon. Further, Patent Document 4 discloses a material for shot processing of molybdenum disulfide that contains approximately 95 mass % of molybdenum disulfide having an average grain size of approximately 1 μm to 20 μm. This material for shot processing containing molybdenum disulfide is shot at a shot processing speed of 100 m/s or more using a shot peening apparatus.
With respect to the aforementioned planetary bearing and the like, in addition to having a structure that makes it difficult to supply lubricating oil, as the result of a decrease in the size of transmissions or the use of CVTs (Continuously Variable Transmissions) and the like in recent years, there are demands for further acceleration of the maximum speed of revolution of the planetary gear (outer member), and this is considered to be accompanied by a rise in the working temperature. Further, with respect to planetary gear needle bearings, faults such as smearing, seizure, abrasion and peeling have become more noticeable than heretofore as a result of downsizing of the planetary gear needle bearings.
Further, the aforementioned ball screws are being required to bear heavier loads, and because the reciprocating motion stroke is also performed in an even shorter period, an oil film is hardly formed when the ball screw rotates in reverse, and occurrence of abrasion, flaking and seizure due to surface damage on the rolling contact surface of the ball, nut and screw shaft have become more noticeable.
Although the technology disclosed in the aforementioned Patent Document 1 is disclosed as technology that prevents smearing, seizure, abrasion, peeling and the like, further lowering of the viscosity of lubricating oil or insufficient oil levels cannot be adequately dealt with by merely forming indentions to form oil film reservoirs. Further, the technology disclosed in Patent Document 2, similarly to the above technology, cannot adequately deal with further lowering of the viscosity of lubricating oil or insufficient oil levels.
Regarding the ball screw, since it is difficult to obtain a solid lubricant effect when a film is not formed evenly, it is not sufficient to merely specify the film thickness as in the technology disclosed in Patent Document 3. Further, adequate performance can also not be obtained by specifying only the shot material as disclosed in Patent Document 4, and it is necessary to accurately specify the film thickness and the coating state.
An object of the present invention is to solve the problems of the prior art as described above, and provide a rolling device in which failures such as smearing, seizure, abrasion and peeling are less liable to occur even when used under conditions of high speed and a heavy load, and which has a long life.
[Patent Document 1]: Japanese Patent No. 2634495
[Patent Document 2]: Japanese Patent No. 2548811
[Patent Document 3]: JP-A-2004-60742
[Patent Document 4]: JP-A-2002-339083